1. Field of the Invention
This invention relates to improvements in circuits and methods for operating polyphase d-c motors and the like, and more particularly to improvements in circuits and methods for preventing parasitic effects on a polyphase d-c motor as it is being powered down.
2. Relevant Background
Polyphase d-c motors have wide applications. For example, one of many areas in which polyphase d-c motors are widely used is in the spinning or rotating of disk drives in personal computer (PC) environments. The disk is spun by the polyphase d-c motor at a relatively precise period of rotation. Typically, a PC disk drive, which may be internal or external to the computer, has a magnetic medium on a disk or platter. One or more magnetic heads are selectively moved radially inwardly or outwardly to enable data to be written to or read from selective locations on the magnetic medium.
When power is removed from the motor, for example, on power off of the PC or on a failure of the power to the system, a complex series of events is often initiated. Such events may include, for example, the running of routines to handle any data that may be in the process of being written to the magnetic media at the time of power off, routines to physically move the magnetic heads to a parked or landing position, routines to stop the rotation of the disk rapidly to avoid excessive wear on the landed heads, and so on.
When the power is removed, typically the power is not instantaneously removed, for example, by a step or ramp function shaped wave form, but instead, is ramped down from the operating voltage to zero. In fact, usually special provisions are taken to ensure that even in a power failure situation, the power to the motor and the circuits associated with the disk drive is conditioned to fall in a known predetermined way after the system power has failed.
After the power has failed or has been removed, however, the motor, which has an inertial mass that tends to keep the motor components of the motor in motion, continues to rotate for some time, until frictional losses stop the rotation, or until the motor is deliberately braked. At some point during the falling value of power supply voltage, the back EMF (BEMF) generated by the coils of the motor begins to supply voltage to the system. The BEMF forces the drive terminals of the motor to go below ground potential. Generally, the magnitude of the voltage to which the BEMF can force the driving terminals below ground is limited by an intrinsic diode, which is a part of the driver power transistors. Typically, for example, the BEMF may swing to about 0.6 volts below ground.
This negative voltage can result in the creation of parasitic devices in and around the power device or driving transistors, or in the other integrated circuits that may be associated with the drive circuits. The parasitic devices may turn on and cause unexpected and/or undesired motor functions, voltages, or operations. Thus, what is needed is a circuit and method to prevent parasitic effects on a d-c motor as it is being powered down. Moreover, the negative voltage wastes BEMF power which otherwise could be used in performing the power-down functions, such as moving and parking the heads, or the like. What is needed, furthermore, is a circuit and method to capture additional BEMF power generated by the motor during its power-down that can be used in performing the various shut-down routines, or for other purposes.